32 



PLANT RESPONSE 



effect of hydrogen gas. The characteristic effect oi depres- 

 sion occurs in this case after a much longer interval than 

 is required by carbonic acid. The immediate result of 

 application is very erratic and various. There may some- 

 times be an exaltation, or even the contrary, a reversal, of 



Fig. 69. Effect of Hydrogen Gas 



[a) Normal response ; {b) after twelve hours' exposure to H. ; 



(r) slow revival of response after readmission of air. 



the normal response. But after a twelve hours' exposure to 

 the action of this gas the responses of the tissue are so much 

 diminished as to approach very near abolition. On now, 

 however, allowing air charged with water-vapour to displace 

 the hydrogen gas, the responses undergo a steady revival 



(fig. 69). There is here no sudden 



exaltation, such as is the general after- 

 effect of carbonic acid gas. 



Effect of carbon disulphide. — 

 We have seen that the depressing 

 effect of hydrogen takes place very 

 slowly. This is owing to the fact 

 that this gas acts here rather as an 

 agent for cutting off that supply of 

 oxygen that is necessary to the 

 Slf "SlpM? t maintenance of the normal life of the 

 plant, than as a direct poison. But 

 we have other gases which are actively 

 toxic, and in such cases the diminution 

 or abolition of response takes place 

 Such an agent may be found in the 



Fig. 70. Photographic Re- 



Abolishing Response 



Normal response, seen to 

 left, abolished by intro- 

 duction of vapour of CSo. 



with greater rapidity. 



vapour of carbon disulphide (fig. 70). 



